The invention generally relates to voltage to current converters and, more particularly, to a voltage to current converter configured with variable transconductance (Gm).
Voltage-to-current converters are implemented in many conventional applications. Related transconducting devices are electronic building blocks characterized by a current output derived from a voltage input in a linear relationship Iout=Gm*Vin, where Gm is transconductance, and where Seimens (S) is the standard unit of Gm in Amps/Volt. Voltage-to-current converters form the basic input structure of instrumentation amplifiers and are used as basic Gm building blocks in frequency shaping devices. Generally, the value of Gm in a conventional voltage to current converter is fixed and is determined by the circuit elements. Thus, the Gm is fixed when the circuit is manufactured. It is possible, however, to vary the Gm using programmable means on a circuit chip. In such a configuration, the Gm could be varied by such programmable means according to a run-time configuration, rather than a predetermined configuration. These devices are used in programmable linear filters, gain amplifiers and other general purpose linear processing elements. Such transconducting devices, being building blocks of fixed and programmable components, have a large impact on the performance of the device in which they are incorporated. Conventional devices are limited in the manner and quality in which the Gm value is set or otherwise programmed. Performance factors such as the accuracy with which the Gm is set, the range over which the Gm is set, the resolution over which the Gm is set, and the linearity of the Gm at a particular point are all factors that affect the performance of the Gm device. Therefore, improving these factors in a Gm device would improve the device""s performance, as well as the component or other device within which it is incorporated. Therefore, there exists a need for Gm devices having improved performance factors. As will be seen below, the invention accomplishes improved performance factors in an elegant manner.
The invention provides a method and apparatus for performing a voltage to current conversion. In particular, the invention provides a voltage to current converter configured to vary its Gm. Such a converter is configured to receive a voltage input signal combined with a reference voltage signal to be converted to a current output. Optionally, the reference voltage signal may be provided by a parabolic impedance network that includes a bank of resistors and a plurality of corresponding current sources. Each current source corresponds to each node between two resistors, and may be varied in order to program changes in the comparator""s Gm. Each resistor and corresponding current source is configured to create an individual reference voltage reference having a value that occurs in a parabolic manner in relation to other voltage references occurring across the impedance network. The converter further includes a plurality of comparators corresponding to the plurality of voltage reference signals. The reference voltage is summed together with an input voltage to an input of each corresponding comparator. Each comparator is configured to receive an enabling signal source for receiving an enabling signal. The enablement signal source is configured to provide a variable input signal to further control the Gm by varying the input current of the converter. The comparator includes a comparator output that outputs a signal when the comparator is enabled.
A circuit embodying the invention may include a plurality of transistor devices, wherein each device includes a pair of transistors. The discussion that follows describes a field effect transistor (xe2x80x9cFETxe2x80x9d) device. The invention, however, is not limited to such a device. For example, a bipolar device may also be used with the base substituting for the gate, the collector substituting for drain and the emitter substituting for the source may be used. Each transistor device has individual gate, source and drain connections, and each of the pair of transistors is configured to receive a separate input at each of their respective source connections. Furthermore, each of the pair of transistors has a commonly connected gate. The other gate of each of the respective transistors is commonly connected with corresponding gates of transistors contained in each of the other transistor devices. The circuit further includes a voltage source configured to provide a reference voltage to the drain of one of the pair of transistors, and each of the other transistor devices also includes a corresponding voltage source at the drain of one of the pairs of the transistors of the respective device. Further, the device includes a current source connected with the gates of the two transistors that are commonly connected. The current source is configured to produce a variable current at the common junction of the two transistors, providing the ability to vary the Gm of the voltage to current converter.
Such a circuit embodying the invention may include a parabolic bank of resistors configured to provide the reference voltage to the gate of one of the pair of transistors. When a voltage is applied to this bank of resistors, a different reference voltage is applied to the gate of one of a pair of transistors of each of the plurality of transistor devices. The parabolic bank of resistors may be further configured to provide a reference voltage in a manner that would produce reference voltages of any desirable character. In this configuration, the reference voltage provided to a first transistor device is of a relatively higher value than the reference voltage provided to an intermediate transistor device. Furthermore, the intermediate transistor device is of a minimum voltage value relative to the other transistor devices.
In an alternative configuration, the reference voltage provided to a first transistor device is of a relatively lower value than the reference voltage provided to an intermediate transistor device. The intermediate transistor device is of a maximum voltage value relative to the other transistor devices. Thus, if the output voltages were presented in graphical form, the graph would be in parabolic form, with a relative minimum or maximum value occurring at the center, depending on whether the parabola is in a concave upward or downward orientation respectively.